Modern boiling water nuclear reactors (BWR's) include in the core region of the reactor a core bypass volume and a fuel channel volume. The fuel channel volume includes bundles of elongated rods or cladding containing the reacting fuel. These bundles are placed within surrounding channels between lower and upper tie plates, and held in a predetermined array by spacers located axially along the bundles. In many such fuel bundle designs, one or more hollow tubes or "water rods" are also positioned in the bundle to provide an additional volume of moderating water. The core bypass volume is exterior of the channels. It is the region in which the control rods control the nuclear reaction and includes additional water moderator for efficient reaction.
Current fuel bundle design has been limited by the need to operate below thermal limits and avoid thermal hydraulic instabilities and coupled nuclear-thermal-hydraulic reactor core instabilities. The stability limits affect the degree to which the fuel assembly can be optimized for minimum fuel cycle costs. Current BWR fuel bundle design also is limited by the need to be able to shut the reactor down in the cold state with any control rod stuck in the fully withdrawn position.
For BWR's having discrete bundles of fuel rods confined within channel enclosed fuel assemblies, improved bundles are disclosed in commonly owned U.S. Pat. Nos. 5,017,332 and 5,112,570. In accordance with the disclosures in these patents, partial length rods (PLR's) can be utilized which extend from the bottom of the channel (within the boiling region) only part way to the top of the channel. In other words, these PLR's are axially shortened with respect to the remaining full length rods (FLR's) and are distributed throughout the fuel bundle, with the preferred disposition being in the interior of the bundle of fuel rods, away from the channel walls. In a preferred arrangement, the PLR's extend upwardly from the bottom of the fuel bundle and terminate at a selected spacer located in the two phase flow region of the fuel bundle assembly. Not all PLR's need be of the same length however. For example, some may terminate at one spacer, while others terminate at another spacer, upstream or downstream of the one spacer. Preferably, the length of PLR's is at least one-half of the total height of the fuel bundle.
During power operation, a steam water mixture is present in the open area above the partial length rods. However, when the reactor is shutdown in the cold state this open area is filled with water. Consequently, the PLR's have a larger effect on moderator-to fuel volume ratio in the cold state than in the hot state, which favorably aids in nuclear design of the fuel. During start-up of the reactor, an improved cold shutdown margin is produced at the top of the fuel assembly due to the increased moderator-to-fuel ratio at the top of the fuel assembly. Shutdown control rod worth is improved due to some of the moderator above the PLR's being near the control rods where the increased moderation increases the number of neutrons that are transported or diffused to the control rod surface. During power reactor operation, the PLR's effectively channel steam flow to the expanded interstitial area between rods overlying the ends of the partial length rods. This enables a high slip ratio of steam with respect to water and increases the density of the moderating water about the remaining rods in the upper region of the bundle at power operation. Additional benefits and advantages accruing from the use of PLR's are set out in the '570 and '332 patents. Despite the advantages of using PLR's, there are situations and conditions when the use of such shortened fuel rods is problematic. For example, when reload bundles are inserted into a reactor core, mismatches in relative fuel bundle flow and reactivity can occur between fresh fuel bundles having some PLR's, and old fuel bundles having all FLR's. Presently, control rod movement is the only way to change power and, to a lesser extent, flow mismatches between fuel bundles of different designs and/or operating lifetimes. This can reduce allowable core operating power, particularly in mixed cores containing both PLR's and FLR's.